1. Field of the Invention
The present invention relates to an engine antifreeze coolant composition which improves the thermal stability of the glycol component of a glycol/water coolant composition in engine cooling/heating systems.
2. Background of the Invention
It is well known to use heat transfer fluids in heat exchanging systems, such as the central heating circuits and engine cooling systems of internal combustion engines and diesel engines. Generally, the heat transfer fluid contacts various metals, alloys and other components that form the different parts of the heat exchanging circuits in these systems. Most typically, coolants that are used in the internal combustion engines and heavy duty diesel engine applications are relied upon to remove the excess heat produced by the internal combustion process.
The coolants, which are also referred to as antifreeze compositions, usually comprise a water-soluble organic fluid to lower the freezing point of the heat transfer fluid. The water-soluble organic fluid is also referred to as an organic freezing point depressant. This water-soluble organic fluid is typically a glycol, for example, monoethylene glycol or monopropylene glycol. Other equivalent glycols can also be used, such as 1,3-butylene glycol, hexylene glycol, diethylene glycol, glycerin, dipropylene glycol and 1,3propanediol. Corrosion inhibitors are also generally added to the antifreeze compositions.
Glycol-based antifreeze compositions are generally diluted with water in order to prepare a ready-to-use aqueous heat transfer fluid. The weight ratio of the amount of the organic freezing point depressant component to that of water in the antifreeze composition is determined by the desired freezing point temperature of the antifreeze composition. Specific combinations of water and organic freezing point depressant components are chosen for desired heat transfer, freezing point and boiling point properties.
Antifreeze coolant compositions also contain additives to prevent corrosion, foaming and scale formation, as well as dyes for fluid identification purposes, and buffering agents to control the pH of the composition.
A current trend in engine manufacture is toward higher efficiency and reduced environmental impact. Higher efficiency can be achieved by increasing power output while reducing engine size and weight. This in turn has the effect of increasing the thermal load to the engine cooling system while often reducing the volume of the cooling system. Such changes result in higher coolant operating temperatures. However, gains in efficiency are often accompanied by increased degradation of the coolant.
The antifreeze coolant composition can degrade in a number of ways. Higher temperatures can accelerate the depletion of the coolant's corrosion inhibitors, prematurely shortening the useful life of the coolant. Thus, corrosion inhibitors which undergo chemical reaction to protect metal surfaces can undergo reaction at accelerated rates at elevated temperatures. For example, nitrite inhibitors added to protect cast iron typically convert to nitrate in use and will convert the iron surfaces to a passivated state. Higher temperatures will accelerate the conversion of nitrite to nitrate, resulting in inhibitor depletion followed by reduced iron surface protection and increased iron corrosion. Ultimately, coolant life is shortened.
Moreover, the coolant base fluid, often composed of glycols can itself degrade to glycol breakdown products such as formate and glycolate through a process of oxidation, perhaps catalyzed by metal surfaces. These oxidation products tend to be acidic and can themselves attack cooling system components. Thus, the presence of glycolates and formates can enhance iron corrosion processes.
Prior art automotive and heavy-duty coolant technology was designed for use at temperatures that typically ranged from about 180-220° F., while heat rejecting surfaces that emanate heat and need to be cooled, such as the engine block, turbo chargers, exhaust gas coolers and fuel injectors, can develop surface temperatures which contact the coolant that range from about 230° F. to about 275° F. As trends continue, it is expected that coolant operating temperatures will increase to greater than 230° F. and that the temperature of the heat rejecting surfaces can be on the order of about 450° F. to about 600° F.
At the temperatures for which they were designed, prior art coolants resist metal corrosion by means of inorganic or carboxylate inhibition. They are also effective to some extent at buffering against the deleterious effects of acidic glycol breakdown products. However, at the anticipated increase in operating temperatures of automotive cooling systems, prior art corrosion protection, inhibitor depletion and glycol stability can be negatively impacted.
U.S. Pat. No. 5,851,419 to Miyake et al discloses an antifreeze composition containing a succinic acid derivative in combination with a benzoic acid derivative to provide improved corrosion protection and greater buffering capacity. Improved buffer capacity is exemplified by titrating the antifreeze composition with acid and noting that increased acid is needed to reduce the pH of the coolant. When glycol degrades to acidic product, the composition will resist pH drop due to enhanced buffer capacity.
U.S. Pat. No. 4,241,016, to Hirozawa discloses a process of inhibiting the corrosion of metals, especially aluminum, using hydroxybenzoic acids as corrosion inhibitors in combination with an organosiloxane silicate copolymer and pH buffering agents capable of buffering in the pH range of 9 to 11.
U.S. Pat. No. 4,460,478, to Mohr et al. discloses a coolant composition containing an orthosilicate ester in a pH range of 6 to 8 containing between 25 to 4000 ppm silicon. Mohr also discloses hydroxybenzoate as a corrosion inhibitor.
U.S. Pat. No. 5,085,793, to Burns et al discloses an antifreeze composition wherein hydroxybenzoates are used for corrosion protection. The antifreeze composition comprises glycol and at least one hydroxyl-substituted aromatic carboxylic acid, having the carboxyl radical proximate to the hydroxyl radical. Also disclosed is a process for inhibiting metal corrosion. The corrosion inhibitor comprises hydroxybenzoate and at least one of borates, silicates, benzoate, nitrates, nitrites, molybdates, thiazoles, and a aliphatic diacid or its salt.
U.S. Pat. No. 5,718,836 to Nakatani discloses a coolant composition containing calcium and/or magnesium salts as well as other corrosion inhibitors, including benzoates.
European patent 0 348 303 discloses improved corrosion protection at elevated temperatures from the addition of salicylate or acetylsalicylate to glycol based antifreeze coolants. The examples show increased pH in comparative coolants when thermally aged due to the formation of basic degradation products. The addition of salicylate appears to repress increase in basicity as indicated by the repressed pH rise during thermal treatment. Repression of metal corrosion is also noted. Although pH rise is undesirable because it indicates metal corrosion, a pH drop also is a matter of concern because an overly acid coolant will itself induce corrosion of metal to which it is exposed. Therefore, preventing the formation of acidic products is an important and different function than prevention of metal corrosion.
U.S. Pat. No. 5,387,360 to Uekusa et al discloses an antifreeze coolant composition comprising glycols as the main constituent, for example, ethylene glycol, propylene glycol, 1,3-butylene glycol, hexylene glycol, diethylene glycol and glycerin. Uekusa's water-free antifreeze composition also includes at least one conventional corrosion inhibitor except silicates, and about 0.005 weight % to about 0.5 weight % of citric acid and/or its corresponding salts.
Uekusa discloses that when an organic acid other than citric acid and its salts, or a tribasic acid, or a dibasic acid is employed in place of citric acid, the resulting coolant has little corrosion inhibiting effect, whether or not the organic acid has a hydroxyl group in the molecule. Uekusa also notes that when the amount of citric acid or its corresponding salts are less than 0.005 weight %, the resulting coolant does not have a satisfactory corrosion preventing effect on metallic materials such as aluminum alloys, resulting in increased weight loss of metallic materials due to corrosion. Uekusa further notes that when the concentration of citric acid or its corresponding salts is greater than 0.5 weight %, the resulting coolant does not have desirable corrosion preventing properties, resulting in increased weight loss of cast aluminum test pieces due to corrosion. The surface of the cast aluminum alloys also turns black. Uekusa does not address the influence or effect of citric acid and its corresponding salts or other additives in a glycol/water antifreeze coolant composition on glycol stability in high temperature applications.